The invention relates to methods for forming photovoltaic (PV) devices. More particularly, the invention relates to methods for forming a transparent oxide layer in a photovoltaic device.
PV (or solar) cells are used for converting solar energy into electrical energy. Typically, in its basic form, thin film solar cells or photovoltaic devices typically include a number of semiconductor layers disposed on a transparent support, where one semiconductor layer serves as a window layer, and a second semiconductor layer serves as an absorber layer. Solar radiation travels through the window layer to the absorber layer, where the optical energy is converted to usable electrical energy. Additional layers are often employed to enhance the conversion efficiency of the PV device.
There are a variety of candidate material systems for PV cells, each of which has certain advantages and disadvantages. Cadmium telluride/cadmium sulfide (CdTe/CdS) heterojunction-based photovoltaic cells are one such example of thin film solar cells.
Typically, a thin layer of transparent conductive oxide (TCO) is deposited between the support and the window layer (for example, CdS) to function as a front contact current collector. However conventional TCOs, such as fluorine-doped tin oxide, indium tin oxide, and aluminum-doped zinc oxide, have high electrical resistivities at the thicknesses necessary for good optical transmission. The use of cadmium tin oxide (CTO) as a TCO provides better electrical, optical, and mechanical properties, as well as stability at elevated temperatures. In addition, for certain configurations, to achieve high device efficiencies with thin CdS films, a thin layer of a buffer material, such as an undoped tin oxide (SnO2) layer, may be intercalated between the cadmium tin oxide (CTO) and the window (CdS) layers.
Typically, to form a transparent CTO layer, a layer of amorphous cadmium tin oxide is deposited on a support, followed by slow thermal annealing of the CTO layer, which is in contact or in close proximity with a CdS film, to achieve desired transparency and resistivity. However, CdS-based annealing of CTO is difficult to implement in a large-scale manufacturing environment. Specifically, it is very difficult to assemble and disassemble the plates before and after the annealing process, typically requiring manual intervention of the operator, and there is a high risk of misalignment that may result in the sublimation of the CTO film. Further, the use of expensive CdS on a non-reusable glass plate for each annealing step increases the cost of manufacturing. The high annealing temperatures (>550° C.) employed for thermal processing of the CTO film, further do not allow for the use of less expensive low softening temperature supports, such as, for example, soda-lime glass.
After crystallization of CTO is achieved, a separate buffer layer (for example, undoped tin oxide) may be deposited on the CTO layer, which may be further followed by a second annealing step to obtain good crystalline quality. The performance of the buffer layer usually depends in part on the crystallinity and morphology of that layer and is affected by the surface of the CTO on which it is deposited. A high quality buffer layer is desirable to obtain the desired performance in the solar cells manufactured therefrom.
Thus, there is a need to reduce the number of steps for depositing and annealing of CTO and optional buffer layers during manufacturing of photovoltaic devices, resulting in reduced costs and improved manufacturing capability. Further, there is a need to provide cost-effective electrodes and photovoltaic devices manufactured using cadmium tin oxide having the desired electrical and optical properties.